1. Field of the Invention
This invention relates generally to the field of Radio Frequency Identification (RFID) tags and labels, and, in particular, to Radio Frequency Identification (RFID) tags and labels that include a self-compensating antenna structure that self-compensates for the material to which it is attached to maintain a substantial impedance match with such material so as to allow efficient performance of the tag.
2. Description of the Prior Art
There is no simple definition of what constitutes an antenna, as all dielectric and conductive objects interact with electromagnetic fields (radio waves). What are generally called antennas are simply shapes and sizes that generate a voltage at convenient impedance for connection to circuits and devices. Almost anything can act to some degree as an antenna. However, there are some practical constraints on what designs can be used with RFID tags and labels.
First, reciprocity is a major consideration in making a design choice. This means that an antenna which will act as a transmitter, converting a voltage on its terminal(s) into a radiated electromagnetic wave, will also act as a receiver, where an incoming electromagnetic wave will cause/induce a voltage across the terminals. Frequently it is easier to describe the transmitting case, but, in general, a good transmit antenna will also work well as a receive antenna (like all rules, there are exceptions at lower frequencies, but for UHF, in the 900 MHz band and above where RFID tags and labels commonly operate, this holds generally true).
Nevertheless, even given the above, it is difficult to determine what is a ‘good’ antenna other than to require that it is one that does what you want, where you want and is built how you want it to be.
However, there are some features that are useful as guides in determining whether or not an antenna is ‘good’ for a particular purpose. When one makes a connection to an antenna, one can measure the impedance of the antenna at a given frequency. Impedance is generally expressed as a composite of two parts; a resistance, R, expressed in ohms, and a reactance, X, also expressed in ohms, but with a ‘j’ factor in front to express the fact reactance is a vector quantity. The value of jX can be either capacitive, where it is a negative number, or inductive, where it is a positive number.
Having established what occurs when one measures the impedance of an antenna, one can consider the effect of the two parts on the antenna's suitability or performance in a particular situation.
Resistance R is actually a composite of two things; the loss resistance of the antenna, representing the tendency of any signal applied to it to be converted to heat, and the radiation resistance, representing energy being ‘lost’ out of the antenna by being radiated away, which is what is desired in an antenna. The ratio of the loss resistance and the radiation resistance is described as the antenna efficiency. A low efficiency antenna, with a large loss resistance and relatively small radiation resistance, will not work well in most situations, as the majority of any power put into it will simply appear as heat and not as useful electromagnetic waves.
The effects of Reactance X are slightly more complex than that for Resistance R. Reactance X, the inductive or capacitive reactance of an antenna, does not dissipate energy. In fact, it can be lessened, by introducing a resonant circuit into the system. Simply, for a given value of +jX (an inductor), there is a value of −jX (a capacitor) that will resonate/cancel it, leaving just the resistance R.
So what is the problem? The problem is bandwidth, frequently described using the term Q (originally Quality Factor). To understand the effect, it is not necessary to understand the mathematics; simply, if an antenna has a value of +jX or −jX representing a large inductance or capacitance, when one resonates this out it will only become a pure resistance over a very narrow frequency band. For example, for a system operating over the band 902 MHz to 928 MHz, if a highly reactive antenna were employed, it might only produce the wanted R over a few megahertz. In addition, high Q/narrow band matching solutions are unstable, in that very small variations in component values or designs will cause large changes in performance. So high Q narrowband solutions are something, in practical RFID tag designs, to be avoided.
An RFID tag, in general, consists of two electrically active parts.    1) The RFID chip, containing rectifiers to generate a DC power supply from the incoming RF signal, logic to carry out the identification function and an impedance modulator, which change the input impedance to cause a modulated signal to be reflected; and,    2) An antenna as described above.
Graphically this arrangement can be represented as two blocks 54, 56 respectively, with two terminals each, as shown in FIG. 4, each with an associated impedance.
If the chip impedance (which tends to be capacitive) and the antenna impedance (which is whatever it is designed to be) are the conjugate of each other, then one can simply connect the chip across the antenna and a useful tag is created. For common RFID chips the capacitance is such that a reasonably low Q adequate bandwidth match can be achieved at UHF frequencies.
However, sometimes it is not so simple to meet operational demands for the tag due to environmental or manufacturing constraints, and then other ways of achieving a good match must be considered. The most common method of maintaining a desired impedance match, is to place between the antenna and chip an impedance matching network. An impedance matching network is usually a network of inductors and capacitors that act to transform both real and reactive parts of the input impedance to a desired level. These components do not normally include resistors, as these dissipate energy, which will generally lead to lower performance.
The problem is shown by describing what can happen to a non-adaptive tag as illustrated in FIG. 5 in a ‘real world’ situation.
FIG. 5 illustrates a simple structure as a half wave dipole 58 on a thin, 100 μm, polyester sheet 60. Each arm 62 is a quarter wavelength long. At 915 MHz in air, this would be 82 mm. The length of the two conductors and their width are set so that the antenna, when the label is held in free space (no dielectric or conductive object within a distance of about 3 m), and the relative dielectric constant of the environment is 1 (air), the impedance of the antenna is a perfect conjugate match to the chip 64. Also assuming that the conductors have a low resistivity and are made of a relatively thick copper, the antenna radiation resistance dominates the resistive part of its impedance. Thus, the antenna has good efficiency. So, when one tries to read this tag by illuminating it at a distance with an RF source, not surprisingly it works quite well, and, at adequate power and frequencies with common RFID chips there is a range of approximately 3 m.
Now if the environment is changed, as shown in FIG. 6, the “perfect” tag described above in FIG. 5, has now been stuck to a block 66 of plastic, 30 mm thick with a dielectric constant of 2.5, and not a dielectric constant of 1 as in air.
Now if one tries to read this tag, the read range is no longer found to be 3 m, but instead 0.5 m.
This change in read range is caused by the fact that the original antenna design was based on the assumption that the antenna was in air having a dielectric constant of 1, and mounted on a very small, thin layer of plastic, which only changes the effective dielectric constant the antenna ‘sees’ by a small amount. So, if one wanted the arms of the antenna to be, one-quarter wavelength long, the following formula would be applied:C (speed of light, approximately 3×108 m/s)=f (operating frequency Hz).λ(wavelength in m)
Now however, stuck to a block of higher dielectric constant material, the antenna is no longer operating in a medium having a dielectric constant of 1. The effective dielectric constant of the block can vary with values between 1 and 2.5. For illustration purposes, let the antenna ‘see’ a dielectric constant of 2. The speed of light c is no longer 3×108 M/s in this medium. It actually reduced by the square root of the relative dielectric constant, and is now 2.12×108. Since c has dropped, at a given frequency, so has the wavelength λ, but the arms of the antenna are still the same length. A quarter wavelength is now approximately 58 mm, but the antenna has elements that are 82 mm in length. Hence the impedance presented to the chip by the antenna will no longer be a conjugate match, and incoming power is lost by reflection, explaining the reduction in read range for the tag.
If the tags were designed to be affixed to 30 mm blocks of plastic, and the blocks always have the same dielectric constant and size, the tags can be made with 58 mm long conducting arms and the range will go back up to near 3 meters.
But what if this is not the case? What if the tags are going to be used with blocks that are always 30 mm thick, but the dielectric constant of the blocks varies from 2 to 7 in an unpredictable way, which cannot be controlled in advance? Sometimes the 58 mm arms design will work very well. Much of the time it will not, as the chip and antenna will be badly mismatched, due to the effective dielectric constant, and hence wavelength, changing.
If each tag could be tuned individually, that is, adjust the arm length and/or add a matching network, consisting of adjustable capacitors and inductors, the tag could be made to work regardless of the dielectric constant of the block, but that would not be practical from a business perspective.
Therefore, for thin, label style tags designed to be attached to products, the performance of the tag when actually deployed on a specific product is an important, if not the most important, critical feature of the device. As discussed above, frequently designers optimize tag performance for ‘free space’, a datum generally given a nominal relative dielectric constant of 1. However, in the real world, the objects the labels are attached to frequently do not have a dielectric constant of 1, but instead have dielectric constants that vary widely. For example, a label having a dipole antenna designed and optimized for ‘free space’ that is instead attached to an object having a dielectric constant that differs from that of ‘free space,’ will suffer a degraded performance, usually manifesting itself as reduced operational range and other inefficiencies as discussed above.
Therefore, while products having differing fixed dielectric constant substrates can be accommodated by changing the antenna design from the ‘free space’ design to incorporate the new dielectric constant, this design change forces the tag manufacturer to produce a broader range of labels or tags, potentially a different type for each target product for which the tag may be applied, hence increasing costs and forcing an inventory stocking problem for the tag manufacturers.
When the tags are to be used on different types of materials that have a range of variable dielectric constants, the best design performance that can be achieved by the tag or label designer is to design or tune the tag for the average value of the range of dielectric constants and accept degraded performance and possible failures caused by significant detuning in specific cases.
The present invention deals with and solves the problems that arise in attempting to design and manufacture an antenna structure for use with an RFID tag or label that is to be mounted on surfaces having a wide range of dielectric constants.
Specifically, while it is unlikely a tag could be made that would perfectly compensate for all values of dielectric constant, the present invention is directed toward meeting the problems that arise in attempting to design and manufacture a tag capable of working on a variety of materials having a range of dielectric constant values, or on different manufacturers' products, to maintain a high performance efficiency for the tag or label.